This invention pertains generally to circuitry for use in digital computers, and particularly to integrated circuitry that may be operated as either an adder or a subtractor in digital computers utilizing large scale integrated (LSI) circuitry.
With the development of LSI, it has become increasingly important that the speed of operation of the major subassemblies in a digital computer be maximized and that such subassemblies be adapted to perform, as required, different arithmetic functions. In U.S. patent application Ser. No. 667,198, filed Nov. 1, 1984 and assigned to the same assignee as this application, an improved adder for 32 bit numbers is disclosed. Such adder, while it is faster than any previously known adder, is best used in applications wherein "full length" numbers are to be added. That is to say, if shorter numbers, say two or four bit numbers, are to be added, the circuitry disclosed in the referenced application would require the same time to operate as would be required to add two 32 bit numbers. Further, the circuitry disclosed in the referenced application may be used only for adding and not for other common procedures, as subtracting numbers.